Hurtling through space at almost 21,500 miles per hour, the previously unknown rock will fly by closer than the moon. It is believed to measure between 22.6 and 49 feet in diameter according to observation news website, The Watchers.

A NASA spokesman told The Express: “This is the first known asteroid to fly by Earth within one lunar distance since two such asteroids flew past us on November 21, and the 52nd this year.”

But don’t head for your nearest underground shelter just yet. Even at its closest, the asteroid is incredibly unlikely to breach the Earth’s atmosphere. Astronomers predict its closest approach won’t get nearer than 0.00150 astronomical units—or about 139,433 miles, to the rest of us.

This is unusually close for a celestial boulder, which NASA considers a “near Earth asteroid.” This designation is reserved for any asteroid closer than six million miles to Earth.

“As of December 24, there are 17,495 known Near-Earth Objects (NEOs) around our planet; 17,389 are asteroids,” the NASA spokesperson said. “This year, we discovered 1,985 new near Earth asteroids. There were 1888 such objects discovered in 2016 and 1,571 in 2015.”

In fact, celestial objects pass by the Earth regularly. However, few strike orbit so close to home. NASA’s Jet Propulsion Laboratory compiles a list of upcoming near-Earth flybys which can help you make sure you build your underground fallout shelter on time. Or, of course, just get out your telescope.

Busy winter skies

This has been a big month for stargazers, with 2017’s only supermoon and the Geminid meteor shower. The Geminids brought us a “potentially hazardous” 3200 Phaethon asteroid, imaged in new detail by the newly-restored Arecibo Observatory Planetary Radar in Puerto Rico. The second-largest single-dish radio telescope in the world was damaged by Hurricane Maria earlier this year.

If astronomers are wrong about 2017 YZ4, and the asteroid is about it hit Earth, we only have a few hours to tunnel underground. The boulder will reach its closest approach at 10:56 ET (7:56 PCT)—give or take six minutes.